Mass dampers—the gigantic weights designed to counteract swaying in skyscrapers—are usually installed during the construction process. But today, a Japanese real estate company announced plans to install six of the devices atop a 39-year-old building in downtown Tokyo. If all goes well, they could pop up on tall buildings all over the world.

The Shinjuku Mitsui Building is Tokyo’s eighth tallest building, and one of its most recognizable, thanks to its 675 feet of stark black glass. It was completed in the mid-1970s, before tuned mass dampers were as common in tall buildings as they are today, but survived smaller earthquakes that have rattled the city since.

But according to CNET and The Japan Times, the building’s developer, Mitsui Fudosan, is no longer content to let fate decide whether the building will weather the next storm. Working with the construction firm Kajima, Mitsui Fudosan is prototyping a mass damper that can be installed on pre-existing buildings. They’ve pledged 5 billion yen (or $51 million) to install six pendulum-like devices—similar to traditional mass dampers—on the building’s roof.

So what, exactly, do these contraptions do? For a good primer on tuned mass dampers, check out this infographic, but here’s the basic gist: Mass dampers are gigantic pendulum-like counterweights that pull a building’s mass in the opposite direction of the prevailing vibrations. So when the ground beneath a foundation moves laterally—either because of wind or seismic activity—the counterweight moves in the opposite direction, taking the structure with it. That’s the essence of the dampers Mitsui Fudosan is planning for their tower, except in this case, the dampers are being installed after the fact. They’ll add more than 300 tons to the building’s weight, but could cut swaying by half, preventing structural damage that could bring a building down during a storm or quake.

Mass dampers have saved buildings from severe ‘quake damage before. During the 2008 Sichuan Earthquake, the gigantic tuned mass damper inside of Taipei 101—the city’s tallest building—could be seen swaying to counteract the vibrations (see footage of it below). It’s an incredible eerie effect, but it could save lives—and buildings. [Japan Times via CNET]